The use of Petri dishes for growing colonies of microorganisms such as bacteria or fungi is well known. A variation on the concept of a Petri dish has developed in recent years, namely, a so-called “contact plate.” A contact plate is a much smaller version of a Petri dish, the dish component of which is provided with a base for grasping the contact plate, and a cover. Contact plates are typically fabricated from polymeric material in mass quantities at a sufficiently low cost as to be disposable after a single use. In use, the convex bottom of the contact plate is filled with microorganism growth medium such as agar, resulting in a convex mound of growth medium. With the lid removed, the contact plate is grasped by the base and the mound of growth medium is pressed against a surface to be tested for bacterial and/or fungal contamination. The lid is then replaced and the contact plate is stored in an environment conducive to microorganism growth. A typical contact plate is pre-loaded with growth medium under sterile conditions and packaged for shipment to the end user.
Two known designs of contact plates are those that are the subject of U.S. Pat. Nos. 5,854,065 and 6,602,704. Both designs have the inherent drawback that the lid and base are held together by a compression fit that is often either too tight to allow ready disengagement between the lid and base or too loose, which can lead to accidental spillage or contamination when handling the contact plate.
What is needed therefore is a lockable contact plate that does not lock except upon application of a specific intentionally applied force, that provides a secure locking engagement between the lid and the base, and which may be readily disengaged from the locking engagement. These needs are met by the present invention, which is summarized and described in detail below.